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On-site drug test kits form a line to indicate the presence or absence of drug in a specimen. A first-time operator typically expects that the drug test will form a line to indicate a positive result. It might seem counter-intuitive, but the opposite is true: the presence of a line indicates a negative result. An understanding of how the kits work may help to explain why the kits are read “backwards.”

Drug test kits typically use a technology called “competitive immunoassay.” Competitive immunoassay describes a process similar to playing musical chairs in which there are more people than chairs. When the music stops one or more people are left standing because all the chairs are occupied. In a drug test kit red-colored reagent antibodies compete with any drug in the specimen to attach to a limited number of receptors on the drug test.

When there is no drug in the specimen to compete with the antibodies, all the receptors are occupied by the red-colored reagent antibodies. When these antibodies bind to the receptors they create a reddish colored line. The presence of a colored line is read as a negative result.

However, if the specimen contains drugs, the drug molecules compete with the antibodies to bind to the receptors, preventing the antibodies from attaching. If the red-colored antibodies cannot attach to the receptors they cannot create the colored line. A test with no line is read as a presumptive positive result.

Our shorthand phrase to describe this process is “The presence of drug in a specimen prevents the line from forming.”